The present invention relates to an acceleration sensor for detecting an acceleration operating on a vehicle, and an acceleration detecting system using the acceleration sensor.
An acceleration sensor is widely used with an occupant protection device such as an airbag and a seat-belt tensioner. Generally, the acceleration sensor is provided on a floor tunnel in a vehicle room together with a control unit, detects an acceleration operating on a vehicle through the floor tunnel, and gives an analog signal representative of the acceleration to the control unit. The control unit decides based on the acceleration signal from the acceleration sensor and a collision decision threshold value whether to drive the occupant protection device, and controls the occupant protection device according to the decision.
By the way, when an impact of collision is absorbed by a crush of a collision part of the car body, it can be assumed that a collision acceleration transmitted to the floor tunnel is weakened. In such a case, an acceleration detected by the acceleration sensor provided on the floor tunnel is small. In particular, a collision, such as an offset collision or an oblique collision, tends to cause a case in which the acceleration transmitted to the floor tunnel is weakened. Because of this, from the viewpoint of providing the acceleration sensor near a collision part, providing the acceleration sensor at a front part of the vehicle can be considered. However, according to this, since the acceleration sensor must be provided near an engine of the vehicle, the acceleration sensor is placed under a state of directly receiving a great variation of temperature. Hence, a conventional acceleration sensor detecting an acceleration as an analog signal can not be provided.
Therefore, an object of the present invention is to provide an acceleration sensor which can be provided near an engine of an automobile.
Another object of the present invention is to provide an acceleration detecting system using the acceleration sensor.
An acceleration sensor in claim 1 comprises: a piezo-electric element for detecting an acceleration; an amplifier circuit for inputting voltage outputs of both ends of said piezo-electric element, and for giving a differential amplification signal, which is obtained by differentially amplifying the voltage outputs of both ends of said piezo-electric element, as a sensor output; a bias resistor circuit provided at an input side of said amplifier circuit; a capacitor inserted in parallel with said piezo-electric element in order to lower a lower cut-off frequency without increasing resistance values of said bias resistor circuit; temperature compensation means for adjusting gain of said amplifier circuit so that an output-temperature characteristic of said piezo-electric element is compensated; and a reference voltage circuit for giving a reference potential to said amplifier circuit and said bias resistor circuit.
According to a composition like this, the output fluctuations of the piezo-electric element due to variations in ambient temperature are compensated by the gain adjustment of the amplifier circuits by means of the temperature compensation means. Accordingly, even when the acceleration sensor is provided in a place with extreme ambient temperature variations such that it directly receives heat from a vehicle engine, the sensor output of the acceleration sensor does not fluctuate by variations in ambient temperature. Also, by inserting the capacitor in parallel with the piezo-electric element, the composite capacity is increased. By this, the lower cut-off frequency can be lowered without increasing resistance values of the bias resistor circuit.
In the acceleration sensor of claim 2 having a connection with the composition of claim 1, a capacity of said capacitor is set so that resistors of said bias resistor circuit become values that can be used in a normal atmosphere and so that the lower cut-off frequency becomes a low value in which velocity variations can easily be detected. By this, the acceleration sensor can give a lower frequency component, and thereby the sensor output that facilitates a collision decision can be given. Also, it is not necessary to set resistors of the bias resistor circuit to such high resistance values that cannot be used in an ordinary atmosphere. Further, migration in the piezo-electric element, occurring by setting the resistors of the bias resistor circuit to high resistance values, can be prevented.
In the acceleration sensor of claim 3 having a connection with the composition of claim 1, said amplifier circuit has a first non-inverting amplifier circuit for non-inversion amplifying one voltage of said piezo-electric element, a second non-inverting amplifier circuit for non-inversion amplifying the other voltage of said piezo-electric element, and a differential amplifier circuit for differentially amplifying outputs of said first and second non-inverting amplifier circuits; and said temperature compensation means is a single temperature compensation element for adjusting gains of said first and second non-inverting amplifier circuits. In the acceleration sensor of claim 4 having a connection with the composition of claim 3, said first non-inverting amplifier circuit has a first operational amplifier, one input terminal of the first operational amplifier being connected to one end of said piezo-electric element, and the other input terminal of the first operational amplifier being connected to an output terminal of the first operational amplifier through a first resistor; said second non-inverting amplifier circuit has a second operational amplifier, one input terminal of the second operational amplifier being connected to the other end of said piezo-electric element, and the other input terminal of the second operational amplifier being connected to an output terminal of the second operational amplifier through a second resistor; and said temperature compensation element is inserted between the other input terminal of the first operational amplifier and the other input terminal of the second operational amplifier, and decreases the gains of the first and second operational amplifiers when temperature rises and increases the gains when temperature drops. According to compositions like these, since the gains of the first and second non-inverting amplifier circuits are adjusted by the single temperature compensation element, decrement in the number of elements and simplification of circuit composition can be achieved.
In the acceleration sensor of claim 5 having a connection with the composition of claim 4, said first and second resistors are set so that a drop in voltage outputs of said piezo-electric element due to the parallel insertion of said capacitor to said piezo-electric element can be supplemented by the gains of said first and second non-inverting amplifier circuits. By this, since the drop in the voltage outputs of said piezo-electric element is supplemented, it is possible to obtain a desired sensor output.
In the acceleration sensor of claim 6 having a connection with the composition of claim 5, said differential amplifier circuit has an operational amplifier, one input terminal of the operational amplifier receiving the output of said second non-inverting amplifier circuit and the reference potential of said reference voltage circuit, and the other input terminal of the operational amplifier receiving the output of said first non-inverting amplifier circuit; and said reference voltage circuit has a reference voltage buffer amplifier for matching with output impedances of said first and second non-inverting amplifier circuits, and gives the reference potential to at least said differential amplifier circuit by way of the reference voltage buffer amplifier. By this, since a common mode rejection ratio of the differential amplifier circuit becomes large, an influence of offset voltage due to the gain increment of the first and second non-inverting amplifier circuits is suppressed by the differential amplifier circuit.
In the acceleration sensor of claim 7 having a connection with the composition of claim 1, a sensor power supply line to which a constant voltage is supplied from outside is further included, an output terminal of said amplifier circuit is grounded through an output resistor, and the sensor output given from said amplifier circuit is output as current variations in said sensor power supply line. Because of this, it is not necessary to provide a signal line. Also, since it is not necessary to use a ground potential by car body grounding as a reference, noise prevention can be done more effectively.
In the acceleration sensor of claim 8 having a connection with the composition of claim 1, a sensor power supply line to which a constant voltage is supplied from outside, a sensor output signal line for outputting the sensor output of said amplifier circuit, and a reference voltage signal line for outputting the reference potential of said reference voltage circuit are further included, and the sensor output given from said amplifier circuit is output as a voltage signal by means of said sensor output signal line and said reference voltage signal line. According to this, even when the voltage of the sensor power supply line fluctuates for some reason or other, since the sensor output of the amplifier circuit and the reference potential of the reference voltage circuit fluctuate together with the voltage of the sensor power supply line, the voltage fluctuation in the sensor power supply line can be cancelled.
In the acceleration sensor of claim 9 having a connection with the composition of claim 1, said amplifier circuit, which has an integrating function, differentially amplifies and integrates the voltage outputs of both ends of said piezo-electric element, and gives an integrated differential amplification signal as the sensor output. In the acceleration sensor of claim 10 having a connection with the composition of claim 9, said amplifier circuit has a first non-inverting amplifier circuit for non-inversion amplifying one voltage of said piezo-electric element, a second non-inverting amplifier circuit for non-inversion amplifying the other voltage of said piezo-electric element, and a differential amplifier circuit for differentially amplifying outputs of said first and second non-inverting amplifier circuits, said first and second non-inverting amplifier circuits and/or said differential amplifier circuit having the integrating function. In the acceleration sensor of claim 11 having a connection with the composition of claim 10, said first non-inverting amplifier circuit has a first operational amplifier, one input terminal of the first operational amplifier being connected to one end of said piezo-electric element, and the other input terminal of the first operational amplifier being connected to an output terminal of the first operational amplifier through a parallel connection of a first resistor and a first capacitor; and said second non-inverting amplifier circuit has a second operational amplifier, one input terminal of the second operational amplifier being connected to the other end of said piezo-electric element, and the other input terminal of the second operational amplifier being connected to an output terminal of the second operational amplifier through a parallel connection of a second resistor and a second capacitor. In the acceleration sensor of claim 12 having a connection with the composition of claim 10, said differential amplifier circuit has a third operational amplifier, one input terminal of the third operational amplifier being connected to an output terminal of said second non-inverting amplifier circuit through a third resistor and being also connected to said reference voltage circuit through a parallel connection of a fourth resistor and a third capacitor, and the other input terminal of the third operational amplifier being connected to an output terminal of said first non-inverting amplifier circuit through a fifth resistor and being also connected to an output terminal of the third operational amplifier through a parallel connection of a sixth resistor and a fourth capacitor. According to compositions like these, since the integrated value of an acceleration is given as the sensor output, it is not necessary to execute an integration processing of the sensor output in a control unit receiving the sensor output. Because of this, a composition of the control unit can be simplified, and a processing speed of control can be risen.
In the acceleration sensor of claim 13 having a connection with the composition of claim 1, the acceleration sensor further comprises: a circuit base board, an acceleration sensor circuit which has said piezo-electric element, said amplifier circuit, said bias resistor circuit, said capacitor, said temperature compensation means and said reference voltage circuit being mounted on said circuit base board; a metallic shielding case having a container shape with an opened upper face, said circuit base board being fixed to the upper face of said shielding case so that a face of said circuit base board having the acceleration sensor circuit becomes inside said shielding case; and a plastic housing having a container chamber which houses said shielding case, wherein said shielding case is formed so that a width of said shielding case in acceleration detecting directions is a little larger than a width of the container chamber of said housing, and side walls of said shielding case meeting at right angle to the acceleration detecting directions pressure-contact with an inside face of said container chamber. According to a composition like this, since the side walls of the shielding case meeting at right angle to the acceleration detecting directions are in the pressure-contact with the container chamber of the housing, an acceleration is directly transmitted from the housing to the shielding case. Because of this, dispersion in a sensor performance due to dispersion in a gap between the shielding case and the container camber can be prevented.
In the acceleration sensor of claim 14 having a connection with the composition of claim 1, the acceleration sensor further comprises: a circuit base board, an acceleration sensor circuit which has said piezo-electric element, said amplifier circuit, said bias resistor circuit, said capacitor, said temperature compensation means and said reference voltage circuit being mounted on said circuit base board; and a plastic housing having a container chamber of which an inside face is covered with a metallic layer for shielding, wherein said circuit base board is fixed to an upper face of said container chamber so that a face of said circuit base board having the acceleration sensor circuit becomes inside said container chamber. According to this, since there is no need of using a metallic shielding case, it is possible to reduce the number of parts and weight of the acceleration sensor.
An acceleration sensor in claim 15 comprises: a piezo-electric element for detecting an acceleration; an amplifier circuit for inputting voltage outputs of both ends of said piezo-electric element, for differentially amplifying and integrating the voltage outputs of both ends, and for outputting an integrated differential amplification signal; a bias resistor circuit provided at an input side of said amplifier circuit; a capacitor inserted in parallel with said piezo-electric element in order to lower a lower cut-off frequency without increasing resistance values of said bias resistor circuit; temperature compensation means for adjusting gain of said amplifier circuit so that an output-temperature characteristic of said piezo-electric element is compensated; a reference voltage circuit for giving a reference potential to said amplifier circuit and said bias resistor circuit; and at least one comparison circuit for inputting the integrated differential amplification signal of said amplifier circuit, for giving a comparison output based on the integrated differential amplification signal and a prescribed threshold value, and for giving the comparison output as a sensor output.
According to a composition like this, similarly to the acceleration sensor of claim 1, even when the acceleration sensor is provided in a place with extreme ambient temperature variations such that it directly receives heat from a vehicle engine, the sensor output of the acceleration sensor does not fluctuate by variations in ambient temperature. Also, the lower cut-off frequency can be lowered without increasing resistance values of the bias resistor circuit. Furthermore, according to the acceleration sensor in claim 15, since a collision signal is directly given by the comparison output, a composition of the control unit receiving the sensor output can be further simplified.
In the acceleration sensor of claim 16 having a connection with the composition of claim 15, a capacity of said capacitor is set so that resistors of said bias resistor circuit become values that can be used in a normal atmosphere and so that the lower cut-off frequency becomes a low value in which velocity variations can easily be detected. By this, the acceleration sensor can give a lower frequency component, and thereby the sensor output that facilitates a collision decision can be given. Also, it is not necessary to set resistors of the bias resistor circuit to such high resistance values that cannot be used in an ordinary atmosphere. Further, migration in the piezo-electric element, occurring by setting the resistors of the bias resistor circuit to high resistance values, can be prevented.
In the acceleration sensor of claim 17 having a connection with the composition of claim 15, said amplifier circuit has a first non-inverting amplifier circuit for non-inversion amplifying one voltage of said piezo-electric element, a second non-inverting amplifier circuit for non-inversion amplifying the other voltage of said piezo-electric element, and a differential amplifier circuit for differentially amplifying outputs of said first and second non-inverting amplifier circuits, said first and second non-inverting amplifier circuits and/or said differential amplifier circuit having the integrating function; and said temperature compensation means is a single temperature compensation element for adjusting gains of said first and second non-inverting amplifier circuits. According to these, since the gains of the first and second non-inverting amplifier circuits are adjusted by the single temperature compensation element, decrement in the number of elements and simplification of circuit composition can be achieved.
In the acceleration sensor of claim 18 having a connection with the composition of claim 17, the gains of said first and second non-inverting amplifier circuits are set so that a drop in voltage outputs of said piezo-electric element due to the parallel insertion of said capacitor to said piezo-electric element can be supplemented by the gains of said first and second non-inverting amplifier circuits. By this, since the drop in the voltage outputs of said piezo-electric element is supplemented, it is possible to obtain a desired sensor output.
In the acceleration sensor of claim 19 having a connection with the composition of claim 18, said reference voltage circuit has a reference voltage buffer amplifier for matching with output impedances of said first and second non-inverting amplifier circuits, and gives the reference potential to at least said differential amplifier circuit by way of the reference voltage buffer amplifier. By this, since a common mode rejection ratio of the differential amplifier circuit becomes large, an influence of offset voltage due to the gain increment of the first and second non-inverting amplifier circuits is suppressed by the differential amplifier circuit.
In the acceleration sensor of claim 20 having a connection with the composition of claim 15, said comparison circuit includes: a comparator for inputting the integrated differential amplification signal of said amplifier circuit and a constant voltage giving the threshold value, for giving a first level signal when the integrated differential amplification signal is below the threshold value, and for giving a second level signal when the integrated differential amplification signal exceeds the threshold value; and a chattering prevention circuit for providing the second level signal as feedback to an input side of said comparator receiving the integrated differential amplification signal when the integrated differential amplification signal exceeds the threshold value. According to this, since chattering of the comparator can be prevented without varying a reference voltage of a comparator, a circuit composition becomes simple.
In the acceleration sensor of claim 21 having a connection with the composition of claim 20, a first comparison circuit with a first threshold value and a second comparison circuit with a second threshold value different from the first threshold value are included as said comparison circuit, and a first and second comparison outputs are given as the sensor output based on comparison between the integrated differential amplification signal and the first and second threshold values. According to this, since a time interval between the first comparison output and the second comparison output becomes small under a high-speed collision and becomes large under a low-speed collision, it is possible to give the sensor output including information representative of an extent of collision. Because of this, it is possible to control an occupant protection device such as an airbag more exactly.
In the acceleration sensor of claim 22 having a connection with the composition of claim 20, a sensor power supply line to which a constant voltage is supplied from outside is further included, said comparison circuit includes a switching element inserted between said sensor power supply line and the ground, and said comparison output is output as current variations in said sensor power supply line by turning On/Off said switching element with said first and second level signals. Because of this, it is not necessary to provide a signal line. Also, since it is not necessary to use a ground potential by car body grounding as a reference, noise prevention can be done more effectively.
In the acceleration sensor of claim 23 having a connection with the composition of claim 15, the acceleration sensor further comprises: a circuit base board, an acceleration sensor circuit which has said piezo-electric element, said amplifier circuit, said bias resistor circuit, said capacitor, said temperature compensation means, said reference voltage circuit and said comparison circuit being mounted on said circuit base board; a metallic shielding case having a container shape with an opened upper face, said circuit base board being fixed to the upper face of said shielding case so that a face of said circuit base board having the acceleration sensor circuit becomes inside said shielding case; and a plastic housing having a container chamber which houses said shielding case, wherein said shielding case is formed so that a width of said shielding case in acceleration detecting directions is a little larger than a width of the container chamber of said housing, and side walls of said shielding case meeting at right angle to the acceleration detecting directions pressure-contact with an inside face of said container chamber. According to a composition like this, since the side walls of the shielding case meeting at right angle to the acceleration detecting directions are in the pressure-contact with the container chamber of the housing, an acceleration is directly transmitted from the housing to the shielding case. Because of this, dispersion in a sensor performance due to dispersion in a gap between the shielding case and the container camber can be prevented.
In the acceleration sensor of claim 24 having a connection with the composition of claim 15, the acceleration sensor further comprises: a circuit base board, an acceleration sensor circuit which has said piezo-electric element, said amplifier circuit, said bias resistor circuit, said capacitor, said temperature compensation means, said reference voltage circuit and said comparison circuit being mounted on said circuit base board; and a plastic housing having a container chamber of which an inside face is covered with a metallic layer for shielding, wherein said circuit base board is fixed to an upper face of said container chamber so that a face of said circuit base board having the acceleration sensor circuit becomes inside said container chamber. According to this, since there is no need of using a metallic shielding case, it is possible to reduce the number of parts and weight of the acceleration sensor.
In the acceleration sensor of claim 25 having a connection with the composition of claim 13 or 14 or 23 or 24, a metallic layer for shielding is formed on a face opposite to the face of said circuit base board having the acceleration sensor circuit. According to this, electromagnetic interference can be prevented more effectively.
An acceleration detecting system in claim 26 comprises: an acceleration sensor provided in a front part of a vehicle, said acceleration sensor having a piezo-electric element for detecting an acceleration, an amplifier circuit for inputting voltage outputs of both ends of said piezo-electric element, and for giving a differential amplification signal, which is obtained by differentially amplifying the voltage outputs of both ends of said piezo-electric element, as a sensor output, bias resistor circuit provided at an input side of said amplifier circuit, a capacitor inserted in parallel with said piezo-electric element in order to lower a lower cut-off frequency without increasing resistance values of said bias resistor circuit, temperature compensation means for adjusting gain of said amplifier circuit so that an output-temperature characteristic of said piezo-electric element is compensated, reference voltage circuit for giving a reference potential to said amplifier circuit and said bias resistor circuit, and a sensor power supply line to which a constant voltage is supplied from outside, said acceleration sensor outputting the sensor output given from said amplifier circuit as current variations in said sensor power supply line; a transmission cable of which one end is connected to said sensor power supply line; and a receiving circuit, being provided in a room of the vehicle, being connected to the other end of said transmission cable, and having a unit power supply line which supplies the constant voltage to said sensor power supply line, said receiving circuit receiving the sensor output of said acceleration sensor by detecting current variations in said unit power supply line.
According to a composition like this, the acceleration sensor is provided in the front part of the vehicle, and the sensor output of the acceleration sensor is received by the receiving circuit provided in the vehicle room. Since the acceleration sensor is provided in the vehicle front, even when a collision acceleration transmitted to a floor tunnel in the vehicle room is weakened, it is possible to detect the collision acceleration quickly. Also, since the sensor output of the acceleration sensor is given as the current variations in the power supply line to the receiving circuit, there is no need of providing a signal line, and thereby simplification of a composition can be achieved. Furthermore, since it is not necessary to use a ground potential by car body grounding as a reference, noise prevention can be done more effectively.
In the acceleration detecting system of claim 27 having a connection with the composition of claim 26, said amplifier circuit of said acceleration sensor, which has an integrating function, differentially amplifies and integrates the voltage outputs of both ends of said piezo-electric element, and gives an integrated differential amplification signal as the sensor output. According to a composition like this, since an integrated value of an acceleration is given as the sensor output, it is not necessary to execute an integration processing of the sensor output in a control unit receiving the sensor output. Because of this, a composition of the control unit can be simplified, and a processing speed can be risen.
An acceleration detecting system in claim 28 comprises: an acceleration sensor provided in a front part of a vehicle, said acceleration sensor having a piezo-electric element for detecting an acceleration, an amplifier circuit for inputting voltage outputs of both ends of said piezo-electric element, for differentially amplifying and integrating the voltage outputs of both ends, and for outputting an integrated differential amplification signal, a bias resistor circuit provided at an input side of said amplifier circuit, a capacitor inserted in parallel with said piezo-electric element in order to lower a lower cut-off frequency without increasing resistance values of said bias resistor circuit, temperature compensation means for adjusting gain of said amplifier circuit so that an output-temperature characteristic of said piezo-electric element is compensated, a reference voltage circuit for giving a reference potential to said amplifier circuit and said bias resistor circuit, at least one comparison circuit for inputting the integrated differential amplification signal of said amplifier circuit, for giving a comparison output based on the integrated differential amplification signal and a prescribed threshold value, and for giving the comparison output as a sensor output, and a sensor power supply line to which a constant voltage is supplied from outside, said acceleration sensor outputting the sensor output given from said comparison circuit as current variations in said sensor power supply line; a transmission cable of which one end is connected to said sensor power supply line; and a receiving circuit, being provided in a room of the vehicle, being connected to the other end of said transmission cable, and having a unit power supply line which supplies the constant voltage to said sensor power supply line, said receiving circuit receiving the sensor output of said acceleration sensor by detecting current variations in said unit power supply line.
According to a composition like this, similarly to the acceleration detecting system of claim 26, even when the collision acceleration transmitted to the floor tunnel in the vehicle room is weakened, it is possible to detect the collision acceleration quickly. Also, there is no need of providing a signal line, and it is not necessary to use a ground potential by car body grounding as a reference. Furthermore, according to the acceleration detecting system of claim 28, since a collision signal is directly given by the comparison output, a composition of the control unit in which the receiving circuit is provided can be further simplified.
In the acceleration detecting system of claim 29 having a connection with the composition of claim 26 or 27 or 28, said receiving circuit has: a current mirror circuit, inserted between said unit power supply line and said transmission cable, for giving a current output according to the current variations due to the sensor output of said acceleration sensor; and a detection resistor, inserted between an output side of said current mirror circuit and the ground, for outputting a voltage signal according to the current output of said current mirror circuit.
In the acceleration detecting system of claim 30 having a connection with the composition of claim 26 or 27 or 28, said receiving circuit has: a transistor, at its base being connected through a first resistor to said unit power supply line and being also connected to said transmission cable, and at its emitter being connected through a second resistor to said unit power supply line, said transistor giving a collector current according to the current variations due to the sensor output of said acceleration sensor; and a detection resistor, inserted between a collector of said transistor and the ground, for outputting a voltage signal according to the collector current of said transistor.
In the acceleration detecting system of claim 31 having a connection with the composition of claim 26 or 27 or 28, said receiving circuit has: a resistor, at its one end being connected to said transmission cable, and at its the other end being connected to said unit power supply line; a constant-current circuit, when a current flowing through said resistor varies by the sensor output of said acceleration sensor, for giving a current output according to variations of the current flowing through said resistor; and a detection resistor, inserted between an output side of said constant-current circuit and the ground, for outputting a voltage signal according to the current output of said constant-current circuit.
An acceleration detecting system in claim 32 comprises: an acceleration sensor provided in a front part of a vehicle, said acceleration sensor having a piezo-electric element for detecting an acceleration, an amplifier circuit for inputting voltage outputs of both ends of said piezo-electric element, and for giving a differential amplification signal, which is obtained by differentially amplifying the voltage outputs of both ends of said piezo-electric element, as a sensor output, a bias resistor circuit provided at an input side of said amplifier circuit, a capacitor inserted in parallel with said piezo-electric element in order to lower a lower cut-off frequency without increasing resistance values of said bias resistor circuit, temperature compensation means for adjusting gain of said amplifier circuit so that an output-temperature characteristic of said piezo-electric element is compensated, a reference voltage circuit for giving a reference potential to said amplifier circuit and said bias resistor circuit, a sensor power supply line to which a constant voltage is supplied from outside, a sensor output signal line for outputting the sensor output of said amplifier circuit, and a reference voltage signal line for outputting the reference potential of said reference voltage circuit; a transmission cable of which one end is connected to said sensor power supply line, said sensor output signal line and said reference voltage signal line; and a receiving circuit, being provided in a room of the vehicle, being connected to the other end of said transmission cable, and having a unit power supply line which supplies the constant voltage to said sensor power supply line, said receiving circuit receiving the sensor output of said acceleration sensor by differentially amplifying the sensor output of said acceleration sensor and the reference potential.
According to a composition like this, similarly to the acceleration detecting system of claim 26, even when the collision acceleration transmitted to the floor tunnel in the vehicle room is weakened, it is possible to detect the collision acceleration quickly. Furthermore, according to the acceleration detecting system of claim 32, even when the voltage of the sensor power supply line fluctuates for some reason or other, since the sensor output of the amplifier circuit and the reference potential of the reference voltage circuit fluctuate together with the voltage of the sensor power supply line, the voltage fluctuation in the sensor power supply line is cancelled by the differential amplification of the receiving circuit.